[Proposed by Gerard Heuvlink] How to map the soil in 3D (and 3D+T) while accounting for huge lateral-vertical and space-time anisotropies and huge differences in measurement support?
We have made tremendous progress in modelling and mapping lateral spatial variation but we have not made nearly as much progress in modelling vertical variation. We are still using fixed depth intervals while we know that soil vertical variation is driven by the development of horizons. We should learn to predict horizon thicknesses and characteristics and how these develop over time. We also have a measurement support problem: in the vertical our measurements are not points but averages over fairly large intervals. Do we take this sufficiently into account when we build and calibrate models of soil variation? Have we ever considered over what depth intervals we should be taking our samples? Should these be 1, 2, 5 or 10 cm thick? Is the answer case-specific? There is still so much to discover in modelling soil vertical variation.
I am not sure it is a first-order challenge. If we remember the soil surveyor’s practices, the values of soil properties per horizons are already averaged since they are measured on composite soil samples constituted by soil samples taken at different depths within the horizon. Most often, the legacy measured profiles we have provide no more than three of these averaged soil properties. It is a much too poor dataset for expecting any improvement in the representation of the soil vertical variations brought by more sophisticated models than our good old spline function. I will advise investing time on this subject when I will see substantial progresses in inexpensive methods for producing better data on soil vertical variations. However, another challenge related to soil vertical variation is to be cited: we are not good in predicting the soil properties of the deepest soil layers, especially the saprolite (between the bottom of the pedogenic horizons and the bedrock). Yet, these soil properties are required by many users dealing with forest or vineyard or with superficial aquifers. We should improve this in the future.